Allow theme.ini to specify external CSS URLs, such as Google Font API loaders.

Allow theme.ini to specify external CSS URLs, such as Google Font API loaders.

Example theme.ini:

  external="http://fonts.googleapis.com/css?family=Lato:100,100italic,300,300italic,400,400italic,700,700italic,900,900italic"
  include=rebase

Notes:
* URLs must be quoted in the .ini file or the .ini file parser explodes!
* To do multiples, list as external[] instead of external.
* If there's an included base theme, any externals it lists will be included first.
* All externals are loaded before any local styles.